1. Field of the Invention
The present invention relates to humidity measurement. The invention describes an instrument, process,and measurement scale for determining absolute moisture content, independent of any measurement of the humidity source temperature. The operating principles are suitable for use in industrial high temperature applications.
2. Description of the Prior Art
The art of psychrometry and specifically humidity measurement continues to be a difficult subject area for both laymen and technical personnel. Misunderstanding of the subject matter is compounded by the use of multiple measurement scales. Relative moisture scales, such as relative humidity and wet bulb temperature, require a secondary measurement of the ambient source temperature. Absolute moisture scales, such as dew point temperature and humidity ratio, are independent of source temperature. Conversion between scales requires extensive psychrometric tables and graphs. Data at elevated temperatures, high humidity levels, and altitudes other than sea level is not widely circulated.
The choice of measurement scale and use of absolute or relative measurements, reflects the application and user expertise. Relative humidity is usually of greatest interest in material processes, as low relative humidity may cause brittleness or problems with static electricity. High relative humidity values may cause an undesirable surface texture, swelling, or clumping regardless of temperature. Meteorologists report relative humidity and the ambient dry bulb temperature in weather data to reflect an environmental comfort level. Historically, wet bulb temperatures have also been measured and reported as a humidity indicator for weather and energy management use. Dew point temperature may be used in a process where dryness of the atmosphere is desirable to avoid condensation at low temperatures. Humidity ratios are typical in drying and other mass transfer processes, where water vapor is an important component in the manufacturing process.
The instrumentation required for humidity measurement has also been largely influenced by the measurement scale of preference. Instruments may be simple and mechanical or sophisticated and electronic. Fully mechanical hygrometers based on material dimensional changes with relative humidity are common. Wet bulb temperature is derived from sling psychrometers or air movement across similar temperature devices clothed in capillary wicking. Optical, chilled mirror systems are available for dew point measurements. Solid state sensors which change capacitance or resistance with relative humidity are increasingly prevalent.
HVAC, meteorological, energy measurement, and light manufacturing industries require humidity measurements and control at ordinary temperatures. Much of the commercially available humidity measuring equipment, measurement scales, and psychrometric literature are directed to these users. There are many commercial and industrial applications which require humidity measurement and control at elevated temperatures. Drying industries, particularly, those in paper, textiles, and building materials, food processors and chemical product manufacturers are representative. These industries operate processes well above 212.degree. F., to 600.degree. F. and higher, and at pressures above and below atmospheric sea level. Consistent and understandable humidity physics and measurements suitable for process control at elevated temperature are required.
To date, no single instrument or humidity measure has proven satisfactory for all applications.